Heretofore, zoom lenses having four lens groups are widely known as general zoom lenses installed on image pickup devices, such as consumer video cameras, surveillance cameras, and the like. The zoom lens includes a first lens group having a positive power, a second lens group having a negative power, a third lens group having a positive power, and a fourth lens group having a positive power arranged in this order from the object side and, when zooming, while the second lens group is moved in a optical axis direction, the fourth lens group is moved to correct a change in image plane position arising from the zooming for focusing. Further, a zoom lens of five lens groups in which a fifth lens group is added as a fixed lens group is also known.
For example, as a zoom lens having a high zoom ratio, a very compact zoom lens is known as described, for example, in Japanese Unexamined Patent Publication Nos. 2007-148340 and 2009-192598.
The zoom lens described in Japanese Unexamined Patent Publication No. 2007-148340 (hereinafter, also referred to as the “zoom lens of conventional example 1”) has a zoom ratio of as high as about 34 times, yet constructed very compact. In the zoom lens of conventional example 1, the second lens group is composed of three negative lenses arranged from the object side and a material having a refractive index greater than 2.0 is disposed to one of the negative lenses to increase the power of the second group. This allows the front lens diameter to be reduced and the amount of movement of the second group to be minimized at the time of zooming, whereby downsizing is achieved.
The zoom lens described in Japanese Unexamined Patent Publication No. 2009-192598 (hereinafter, also referred to as the “zoom lens of conventional example 2”) is also constructed very compact with less number of lenses even though it has a high zoom ratio of about 39 times. The zoom lens of conventional example 2 is downsized by disposing a material having a refractive index of greater than 2.0 to the lens closest to the object side in the second lens group.
Further, for example, zoom lenses having large magnification ratios of, for example, about 19 times yet compact are known as described, for example, in Japanese Unexamined Patent Publication Nos. 2008-158418 and 2008-164725. Hereinafter, the zoom lens described in Japanese Unexamined Patent Publication No. 2008-158418 is also referred to as the “zoom lens of conventional example 3”, and the zoom lens described in Japanese Unexamined Patent Publication No. 2008-164725 is also referred to as the “zoom lens of conventional example 4”.
In each of the zoom lenses of conventional examples 3 and 4 which are of four-group configuration and five-group configuration respectively, the first lens group has a similar configuration in which a cemented lens of negative and positive lenses is disposed closest to the object side and one or two positive lenses are additionally provided.
Further, a zoom lens in which, though a zoom ratio is about 10 times, a material having an Abbe number greater than 30 is used for a negative lens in the first lens group is also known as described, for example, in Japanese Unexamined Patent Publication No. 2003-098434 (hereinafter, also referred to as the “zoom lens of conventional example 5).